0 1020304050
10
AI O II
AI O powder
8
200
0.8
6
150
0.6
4
100
0.4
2
0.2
50
0
0 1020304050
0.6
PEI (%)
BET SA
Pore volume
0.4
increase in storage capacity in the presence of moisture. As shown in Figure 3, however, no effect of moisture on the desorption temperature (i.e. the stability of the adsorbed species) was found. Figure 3 shows two runs, one for dry and one for humid conditions. Results indicate that temperature ramps under the same conditions are very reproducible, and differences in peak shape originate from sample properties. For most samples, the presence or absence of steam mainly affects the height of the release peak, and no significant shift in the release temperature is observed (vertical lines indicate the location of the peak maximum). Effect of PEI loading and support type Consistent with expectations, CO2 capacity increased with increasing PEI content at low sample loadings. For all selected support materials, however, increasing the PEI content beyond a certain material-specific threshold resulted in a dramatic loss of storage capacity (Figures 2 and 3). It has been observed by other authors ( Hoffman et al., 2024 ) that amine utilisation or efficiency is coverage-dependent and decreases with increasing amine content. The authors attribute this to a diffusion limitation in the kinetics of CO2 capture. Results confirm this decay in utilisation and corroborate that excessive polymer addition causes pore blocking and renders substantial fractions of the amine groups in the polymer inaccessible for CO 2 molecules, thus causing a decline in storage capacity (see Figure 4 ). This is also supported by the observed loss of specific Figure 5 BET surface area (blue, left axis) and average pore volume (green, right axis) for selected samples
0.2
0.0
10
20
30
40
50
PEI (%)
Al O I
Al O II
Al O powder
measurements, 50% of the total amount absorbed within two hours was stored within the first 20 minutes. Effect of moisture The impact of moisture on DAC has been reported in the literature. According to current understanding of the storage mechanism in amine-based solid adsorbents, a mobile proton is involved to form either carbamate or carbamic acid. In density functional theory (DFT) calculations, it could be demonstrated that a second amine group is required as an intermediate acceptor for this proton, forming an ammonium ion ( Goeppert et al., 2011; Said et al., 2020 ). In the presence of steam, water can serve as a base, facilitating the proton transfer via a hydronium ion intermediate. Consequently, the presence of steam generally has a positive impact on storage capacity for amine-type adsorbents. This contrasts with the physisorption mechanism observed for zeolite-type sorbents (B oer et al., 2023 ), where competitive adsorption between H 2 O and CO 2 is observed, and the presence of humidity has a detrimental impact on CO 2 capture capacity. In the current study, PEI-modified carriers were used. All of them showed a moderate Figure 4 Total amount of CO 2 stored in the sample (bottom) and the corresponding utilisation of N atoms in the sample (top) as function of PEI content
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